Surface mount type crystal oscillator

ABSTRACT

A surface mount crystal oscillator comprises a crystal blank, an IC chip having an oscillation circuit integrated thereon, and a hermetic package for accommodating the crystal blank and IC chip therein. The hermetic package comprises a substantially rectangular ceramic substrate formed with a metal film which makes a round on one main surface thereof, and a concave metal cover having an open end face bonded to the metal film. The IC chip is secured to the one main surface of the ceramic substrate through ultrasonic thermo-compression bonding using bumps, the crystal blank is disposed above the IC chip, and the ceramic substrate has the one main surface formed as a flat surface.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 11/606,511filed Nov. 30, 2006, now U.S. Pat. No. 7,602,107 issued Oct. 13, 2009,which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface mount type quartz crystaloscillator, and a surface mount type crystal device.

2. Description of the Related Art

Surface mount type crystal oscillators, which have a quartz crystalelement and an oscillation circuit using this crystal element, bothcontained in a surface mount package, are widely used as referencesources for frequency and time in compact portable electronic devicesincluding, among others, portable telephones, because of their smallsize and light weight. An example of such surface mount type crystaloscillators is disclosed in US 2005/0193548A1.

FIG. 1 is a cross-sectional view illustrating an exemplary configurationa conventional surface mount type crystal oscillator. The surface mounttype crystal oscillator has IC (integrated circuit) chip 2 and crystalblank 3 hermetically sealed in package 1. Package 1 comprises ceramicsubstrate 4 as a mount substrate on which components are mounted, andmetal cover 5. Ceramic substrate 4 comprises a laminate of first ceramiclayer 4 a having a substantially rectangular and flat shape, and secondceramic layer 4 b which has a substantially rectangular opening. Theopening formed through second ceramic layer 4 b defines a recess in onemain surface of ceramic substrate 4. On a surface on a laminated side offirst ceramic layer 4 a, which corresponds to the opening of secondceramic layer 4 b, stated another way, on the bottom surface of therecess in ceramic substrate 4, a plurality of circuit terminals 6 aredisposed for electric connection with IC chip 2. On the back surface offirst ceramic layer 4 a, i.e., the outer bottom surface of package 1,mounting electrodes 7 are formed at four corners for use in mounting thecrystal oscillator on a wiring board. Each mounting electrode 7 is alsoprovided with an end face electrode 7 b, i.e., an area formed to extendto a side surface from the outer bottom surface of first ceramic layer 4a. End face electrode 7 b is formed by through-hole processing whenceramic substrate 4 is formed by laminating and burning ceramic sheets.Each mounting electrode 7 is electrically connected to circuit terminal6 through end face electrode 7 b associated therewith, and conductivepaths, not shown, formed on laminated surface of first and secondceramic layers 4 a, 4 b. A soldering fillets are formed on end faceelectrodes 7 b when the crystal oscillator is mounted on the wiringboard by reflow soldering.

Sealing metal film 9 is disposed on the top surface of second ceramiclayer 4 b along the outer periphery thereof. A pair of crystal holdingterminals 10 are formed on the top surface of second ceramic layer 4 balong one side of the opening at positions corresponding to both ends ofthe side for holding crystal blank 3. Crystal holding terminals 10 areelectrically connected to circuit terminals 6 through conductive paths,not shown, formed on ceramic substrate 4.

As illustrated in FIG. 2, crystal blank 3 is, for example, asubstantially rectangular AT-cut quartz crystal blank which is providedwith excitation electrodes 13 on both main surfaces thereof, andlead-out electrodes 14 are extended from a pair of excitation electrodes13 toward opposite ends on one side of crystal blank 3. Both ends of theside of crystal blank 3, to which lead-out electrodes 14 are extended,are secured to crystal holding terminals 10 by conductive adhesive 15 orthe like, thereby electrically and mechanically connecting crystal blank3 to ceramic substrate 4.

IC chip 2, which is substantially rectangular in shape, comprises atleast an oscillation circuit, which uses crystal blank 3, integrated ona semiconductor substrate. The oscillation circuit is formed on one mainsurface of the semiconductor substrate through a general semiconductordevice fabrication process. Therefore, a circuit forming surface refersto one of both main surfaces of IC chip 3, on which the oscillationcircuit is formed, on the surface of the semiconductor substrate. Thecircuit forming surface is also formed with a plurality of IC terminals8 corresponding to the aforementioned circuit terminals 6, asillustrated in FIG. 3. These IC terminals 8 include a power supplyterminal, a ground terminal, an oscillation output terminal, a pair ofconnection terminals for connecting to crystal blank 3, an AFC terminalfor receiving an automatic frequency control (AFC) signal, and the like.Then, IC terminals 8 are bonded to circuit terminals 6 throughultrasonic thermo-compression bonding using bumps 12, thereby securingIC chip 2 within the recess of ceramic substrate 4. This also causes theconnection terminals of IC chip 2 to electrically connect to crystalholding terminals 10, and the power supply, output, ground, and AFCterminals electrically connected to mounting electrodes 7 associatedtherewith.

Metal cover 5 is formed in a concave shape, such that its opening endface is bonded to metal film 9 on second ceramic layer 4 b, for example,through thermo-compression bonding using brazing metal 11 made, forexample, of AuSn (gold-tin) eutectic alloy or the like, thereby bondingmetal cover 5 to ceramic substrate 4. In this way, metal cover 5hermetically seals IC chip 2 placed in the recess of ceramic substrate4, and crystal blank 3 secured to ceramic substrate 4 within package 1.

In such a surface mount type crystal oscillator, since concave metalcover 5 is bonded to the outer periphery of ceramic substrate 4, theinternal volume of package 1 can be made larger, and conversely, thecrystal oscillator can be reduced in size while the internal volume ofpackage 1 is maintained constant.

FIG. 4 illustrates a crystal oscillator which employs a flat metalcover. This crystal oscillator employs package body 1 made of laminatedceramics and having a step in a recess, and flat metal cover 5 a, wheremetal cover 5 a is bonded to package body 1 a to hermetically seal ICchip 2 and crystal blank 3 within the recess. IC chip 2 is secured tothe bottom surface of the recess, while crystal blank 3 has its one sidesecured to the top surface of the step in the recess and is thereby heldin the recess. In such a crystal oscillator, from a viewpoint ofmanufacturing and the like, frame width d2 of the topmost layer of thelaminated ceramic layers in package body 1 must be equal to or largerthan the height of the layer, thus causing this frame width d2 to benecessarily larger. Frame width d2 is, for example, 0.35 mm. Here, theframe width refers to the distance from an inner wall surface opposingthe recess or opening to an outer wall surface in the ceramic layerhaving a recess or opening. Since crystal blank 3 is surrounded by thetopmost layer of the laminated ceramic layers, package body 1 a resultsin having outer dimensions larger than the size of crystal blank 3further by a factor of two or more of frame width d2 in both of verticaland horizontal directions in the figure. On the other hand, when aconcave metal cover as illustrated in FIG. 1 is used, a width needed tobond metal cover 5 to ceramic substrate 4 must only be ensured aroundcrystal blank 3. Typically, this width is equal to thickness d1 of ametal plate which constitutes metal cover 5. This thickness is, forexample, 0.08 mm. As such, the configuration illustrated in FIG. 1 canlargely increase the internal volume of the package in the same outerdimensions and hence largely reduce the crystal oscillator in outerdimensions, as compared with the configuration illustrated in FIG. 4.

For manufacturing the surface mount type crystal oscillator configuredas illustrated in FIG. 1, unburned ceramic sheets (green ceramic sheets)generally having a size corresponding to a plurality of the crystaloscillators are used. The ceramic sheets are then laminated and burned,and then are cut, after burning, into a plurality of ceramic substrates4 each corresponding to one crystal oscillator. Specifically, asillustrated in FIG. 5A, second ceramic sheet 4B having an opening foreach crystal oscillator is laminated on flat first ceramic sheet 4Awhich has been previously formed with an electrode pattern for eachcrystal oscillator, and two ceramic sheets 4A, 4B are laminated andburned together, and then are divided along division line X-X to produceindividual ceramic substrates 4.

In this event, as illustrated in FIG. 5B in an enlarged view,wedge-shaped division groove 16 is cut into each of both main surfacesof the laminate of the ceramic sheets at the position of the divisionline, followed by the burning. Such division grooves 16 thus formedfacilitate the division of the laminate of the ceramic sheets intoceramic substrates 4 for respective crystal oscillators after theburning. First ceramic sheet 4A corresponds to first ceramic layer 4 a,while second ceramic sheet 4B corresponds to second ceramic layer 4 b.

However, when the ceramic sheets are baked after they have been formedwith division grooves 16 as described above, a contractive forceassociated with evaporation of a binder from the ceramic sheetsconcentrates particularly on upper ends of a frame portion defined bysecond ceramic layer 4 b, resulting in external force P exerted in adirection to reduce the area of the opening formed through secondceramic layer 4 b. Such external force P thus produced causes firstceramic layer 4 a to curve into a concave form in burned ceramicsubstrate 4. Resulting ceramic substrate 4 suffers from an exacerbatedflatness, i.e., plane accuracy on the bottom of the recess.

Assuming here that IC chip 2 is secured to the bottom of the recess inceramic substrate 4, which is a mounting substrate, i.e., the surface offirst ceramic layer 4 a, through ultrasonic thermo-compression bondingusing bumps 12, sufficient pressure is not applied to bumps 12 on theconcave surface which lacks the flatness, as illustrated in FIG. 5C,resulting in a lower strength exerted by bumps 12 for securing IC chip 2to ceramic substrate 4. Particularly, in this event, those bumps 12which were not applied with sufficient pressure can compromise electriccontacts, and be more susceptible to peeling when an impact is appliedthereto. In this connection, the flatness of the bottom surface of therecess in ceramic substrate 4 is preferably in a range of 10 to 15 μm orless.

Further, the surface mount type crystal oscillator configured asillustrated in FIG. 1 also has a problem of difficulties in reducing themanufacturing cost because it is fabricated by laminating second ceramiclayer 4 b having an opening on flat first ceramic layer 4 a.Particularly, since a stamping process is required to form the openingthrough second ceramic layer 4 b, the surface mount type crystaloscillator has a problem of a high processing cost. Another problem liesin that a limited area is merely provided for accommodating IC chip 2because the surface mount type crystal oscillator receives IC chip 2within the recess of ceramic substrate 4, and both ends on one side ofcrystal blank 3 are secured to the position of an edge of the recess.When a temperature compensation mechanism is additionally integrated onthe IC chip in addition to the oscillation circuit in order to increaseadded values of the crystal oscillator, the IC chip is also increased insize. Accordingly, a certain area required on the top surface of ceramicsubstrate 4 for securing the crystal blank thereto constitutes a factorof impeding a reduction in size of ceramic substrate 4.

Surface mount type crystal devices which have a concave metal coverbonded to a ceramic substrate are not limited to the crystal oscillatordescribed above. Such a concave metal cover bonded to a ceramicsubstrate can be employed as well in a crystal unit which has a crystalblank encapsulated in a package. Such a surface mount crystal unit isalso utilized as a reference source for frequency and time in portableelectronic devices.

FIG. 6 illustrates an exemplary configuration of a conventional surfacemount type crystal unit. In the illustrated crystal unit, crystal blank3 similar to that illustrated in FIG. 2 is mounted on flat ceramicsubstrate 4 which is substantially rectangular in shape, and concavemetal cover 5 is bonded to ceramic substrate 4 to hermetically sealcrystal blank 3 within a space surrounded by ceramic substrate 4 andmetal cover 5. Ceramic substrate 4 may be, for example, in a two-layerstructure, where metal film 9 is formed on the top surface of a secondlayer along the outer periphery thereof. A pair of crystal holdingterminals 10 are also disposed on the top surface of the second layer.Mounting electrodes 7 c are formed on the bottom surface of ceramicsubstrate 4, i.e., the lower surface of a first layer for use inmounting the crystal unit on a wiring board. Crystal holding terminals10 are electrically connected to mounting electrodes 7 c throughconductive paths formed on laminated surface of the first and secondlayers, and a conductive film formed on the end face of the first layer.Then, both ends on the one side of crystal blank 3, to which lead-outelectrodes 14 extend, are secured to crystal holding terminals 10 withconductive adhesive 16 in a manner similar to the aforementioned.

Metal cover 5 is processed such that an opening end face has a flange,and is bonded to ceramic substrate 4 by thermo-compression bondingthrough the intervention of eutectic alloy 11 such as AuSn alloy or thelike between the flange plane and metal film 9. The flange of metalcover 5 is used to increase the width of a portion of metal cover 5which is bonded to ceramic substrate 4, i.e., a so-called seal-path toensure the bonding strength and air-tight sealing.

The crystal unit illustrated in FIG. 6 can be manufactured at a reducedcost by virtue of the employment of flat ceramic substrate 4, ascompared with a crystal unit (see FIG. 7) which comprises crystal blank3 placed in a recess of package body 1 a, and flat metal cover 5 a forclosing the recess. In this connection, the crystal unit illustrated inFIG. 7 comprises metal ring 11 a for seam welding disposed on the topsurface of package body 1 a to surround a recess, such that metal cover5 a is bonded to metal ring 11 a by seam welding.

In the crystal unit illustrated in FIG. 6, eutectic alloy 10 is meltedby heating, with an opening end face of metal cover 5 remaining incontact with eutectic alloy 10, to bond metal cover to ceramic substrate4. In this event, metal cover 5 is susceptible to shift in position toceramic substrate 4, where part of the flange of metal cover 5, forexample, can protrude from the outer periphery of ceramic substrate 4 toreduce the seal-path, possibly resulting in exacerbated bonding strengthand air-tight sealing. Also, with portion of the flange thus protrudingfrom ceramic substrate 4, the resulting crystal unit can experience suchproblems as a failure in satisfying dimensional criteria related to theexternal shape, and a defective appearance.

Various problems caused by the shift in position between metal cover 5and ceramic substrate 4 can also be experienced in the surface mounttype crystal oscillator as illustrated in FIG. 1.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a surface mount typecrystal device which facilitates a reduction in size and is manufacturedat a lower cost.

It is another object of the present invention to provide a surface mounttype crystal device which is capable of increasing a positioningaccuracy of a metal cover to a ceramic substrate.

It is a further object of the present invention to provide a surfacemount type crystal oscillator which facilitates a reduction in size andis manufactured at a lower cost.

It is a further object of the present invention to provide a surfacemount type crystal oscillator which is capable of increasing apositioning accuracy of a metal cover to a ceramic substrate.

It is a further object of the present invention to provide a surfacemount type crystal oscillator which excels in the flatness of a ceramicsubstrate to ensure the boding of an IC chip through ultrasonicthermo-compression bonding using bumps.

According to a first aspect, the present invention provides a surfacemount type crystal oscillator which comprises a crystal blank, an ICchip having at least an oscillation circuit using the crystal blank,integrated thereon, and a hermetic package for accommodating the crystalblank and the IC chip therein, wherein the hermetic package comprises aceramic substrate having a substantially rectangular shape and formedwith a metal film which makes a round on one main surface thereof, and aconcave metal cover having an open end face bonded to the metal film,the ceramic substrate comprises mounting electrodes at corners on theother main surface thereof for use in mounting the crystal oscillator ona wiring board, the IC chip is secured to the one main surface of theceramic substrate through ultrasonic thermo-compression bonding usingbumps, the crystal blank is disposed above the IC chip, and the one mainsurface of the ceramic substrate is formed as a flat surface.

In the configuration described above, since the ceramic substrate issubstantially flat without recess, the ceramic substrate is preventedfrom curvature associated with a non-uniform contractive force duringburning, thus maintaining the flatness of the ceramic substrate afterthe burning. As a result, the IC chip can be reliably bonded to theceramic substrate through ultrasonic thermo-compression bonding usingbumps. In such a crystal oscillator, the ceramic substrate may becomposed of a first ceramic layer formed with the mounting electrodes,and a second ceramic layer laminated on the first ceramic layer forsecuring the IC chip thereto, wherein the first ceramic layer maycomprise a cutout portion in a central area on at least one sidethereof, the cutout portion having an outer periphery open to the side,and an adjustment terminals is formed on a laminated surface of thesecond ceramic layer exposed by the cutout portion. The adjustmentterminal typically includes a write surface terminal used to writetemperature compensation data into a temperature compensation mechanismwithin the IC-chip, and/or a crystal test terminal used to measureoscillation characteristics of the crystal blank.

According to a second aspect, the present invention provides a surfacemount type crystal oscillator which comprises a substantiallyrectangular crystal blank an IC chip having at least an oscillationcircuit using the crystal blank, integrated thereon, and a packagehaving at least a substantially rectangular ceramic substrate andadapted to accommodate the crystal blank and the IC chip therein,wherein the ceramic substrate comprises mounting electrodes at cornerson a surface thereof which is an outer bottom surface of the package foruse in mounting the crystal oscillator on a wiring board, the ceramicsubstrate comprises a first ceramic layer formed with the mountingelectrodes, and a second ceramic layer laminated on the first ceramiclayer for securing the IC chip thereto, and the first ceramic layercomprises a cutout portion in a central area on at least one sidethereof, where the cutout portion has an outer periphery open to theside, and adjustment terminal is formed on a laminated surface of thesecond ceramic layer exposed by the cutout portion.

According to this configuration, since a write surface terminal forwriting, for example, temperature compensation data is formed in thecutout portion, a probe for writing is more readily accessed to thewrite surface terminal, as compared with a terminal disposed in a holeformed through a center area of a back surface of a substrate, as shownin Japanese Patent Laid-open application No. 8-307153 (JP, A, 8-307153),by way of example. In this event, the ceramic substrate itself can bemade substantially flat, thus making it possible to prevent the ceramicsubstrate from being curved in association with burning.

Here, the thickness of the first ceramic layer, i.e., a difference inlevel between the mounting electrodes and adjustment terminals ispreferably, for example, in a range of 50 μm to 200 μm, and morepreferably in a range of 80 μm to 100 μm.

According to a third aspect, the present invention provides a surfacemount type crystal oscillator which comprises a substantiallyrectangular crystal blank, an IC chip having at least an oscillationcircuit using the crystal blank, integrated thereon, and a hermeticpackage having at least a substantially rectangular ceramic substrateand adapted to accommodate the crystal blank and the IC chip therein,wherein the hermetic package comprises a ceramic substrate having asubstantially rectangular shape and formed with a metal film which makesa round on one main surface thereof, and a concave metal cover having anopen end face bonded to the metal film, the crystal blank comprisesexcitation electrodes respectively formed on a pair of main surfacesthereof, and a pair of lead-out electrodes extended from the excitationelectrodes to both ends on one side of the crystal blank, the ceramicsubstrate has at least the one main surface formed as a flat surface,the ceramic substrate comprises mounting electrodes at corners on theother main surface thereof for use in mounting the crystal oscillator ona wiring board, the IC chip has its circuit forming surface secured tothe one main surface of the ceramic substrate through ultrasonicthermo-compression bonding using bumps, the crystal blank is held in thehermetic package by securing both ends on the one side thereof to a backsurface of the IC chip, and the crystal blank is electrically connectedto a relay terminal formed on the one main surface of the ceramicsubstrate using wire bonding.

According to a fourth aspect, the present invention provides a surfacemount crystal device which comprises a crystal blank and a hermeticpackage for accommodating at least the crystal blank therein, whereinthe hermetic package comprises a ceramic substrate having asubstantially rectangular shape and formed with a metal film which makesa round on a surface thereof, and a concave metal cover having an openend face bonded to the metal film, and the ceramic substrate includes astep for positioning the metal cover on an inner peripheral side of themetal film on the surface of the ceramic substrate.

Crystal devices of the present invention include, for example, a crystalunit which has a crystal blank placed in a package, and a crystaloscillator which has a crystal element and an oscillation circuitintegrated therein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating an exemplary configurationof a conventional surface mount type crystal oscillator;

FIG. 2 is a top plan view illustrating a circuit forming surface in anIC (integrated circuit) chip;

FIG. 3 is a top plan view illustrating an exemplary configuration of acrystal blank;

FIG. 4 is a cross-sectional view illustrating another example of aconventional surface mount type crystal oscillator;

FIG. 5A is a cross-sectional view of unburned ceramic sheets;

FIG. 5B is a partially enlarged cross-sectional view of a portionsurrounded by a circle in FIG. 5A;

FIG. 5C is a cross-sectional view illustrating ceramic layers on whichan IC chip is mounted;

FIG. 6 is a cross-sectional view illustrating an exemplary configurationof a conventional surface mount type crystal unit;

FIG. 7 is a cross-sectional view illustrating another example of aconventional surface mount type crystal unit;

FIG. 8 is a cross-sectional view illustrating the configuration of asurface mount type oscillator according to a first embodiment of thepresent invention;

FIG. 9 is a top plan view of a ceramic substrate in the crystaloscillator illustrated in FIG. 8;

FIG. 10 is a cross-sectional view illustrating a surface mount typecrystal oscillator according to a second embodiment of the presentinvention;

FIG. 11 is a top plan view of a first ceramic layer in the crystaloscillator illustrated in FIG. 10;

FIG. 12 is a bottom view of the crystal oscillator illustrated in FIG.10;

FIG. 13 is a top plan view of a first ceramic layer in a surface mounttype crystal oscillator according to a third embodiment of the presentinvention;

FIGS. 14A and 14B are bottom views each illustrating an exemplaryplacement of terminals in the crystal oscillator of the thirdembodiment;

FIG. 15 is a cross-sectional view illustrating the configuration of asurface mount type crystal oscillator according to a fourth embodimentof the present invention;

FIG. 16 is a cross-sectional view illustrating the configuration of asurface mount type crystal oscillator according to a fifth embodiment ofthe present invention;

FIG. 17A is a cross-sectional view illustrating the configuration of asurface mount type crystal oscillator according to a sixth embodiment ofthe present invention;

FIG. 17B is a top plan view illustrating an auxiliary substrate;

FIGS. 18A to 18C are cross-sectional views each illustrating a surfacemount type crystal unit based on the present invention;

FIGS. 19A and 19B are cross-sectional views each for describing a methodof bonding a metal cover to a ceramic substrate; and

FIG. 20 is a cross-sectional view of a surface mount type crystaloscillator based on the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 8 which illustrates a surface mount type quartz crystaloscillator according to a first embodiment of the present invention, thesame components as those in FIG. 1 are designated the same referencenumerals as those in FIG. 1.

The crystal oscillator illustrated in FIG. 8 is similar to thatillustrated in FIG. 1 in that concave metal cover 5 is bonded tosubstantially rectangular ceramic substrate 4 to encapsulate IC chip 2and quartz crystal blank 3 within hermetic package 1 made up of ceramicsubstrate 4 and metal cover 5. Metal film 9 is disposed on the topsurface of ceramic substrate 4 along the outer periphery thereof, suchthat metal cover 5 has its opening end face bonded to metal film 9 bybrazing metal 11, and is thereby bonded to ceramic substrate 4. Crystalblank 3 used herein is an AT-cut quartz crystal blank similar to thatshown in FIG. 3. IC chip 2, which is similar to that shown in FIG. 2,has an oscillation circuit using crystal blank 3, and a temperaturecompensation mechanism for compensating crystal blank 3 for itstemperature-frequency characteristics, both of which are integratedthereon, and also comprises eight IC terminals on its circuit formingsurface. The IC terminals include a pair of write terminals for writingtemperature compensation data into the temperature compensationmechanism, in addition to the aforementioned power supply terminal,ground terminal, oscillation output terminal, pair of connectionterminal, and AFC terminal. Eight circuit terminals 6, later described,are disposed on the top surface of ceramic substrate 4 in correspondenceto the IC terminals, such that IC chip 2 is mounted on the top surfaceof ceramic substrate 4 by bonding the IC terminals to associated circuitterminals 6 through ultrasonic thermo-compression bonding using bumps12.

The crystal oscillator of the first embodiment differs from the oneillustrated in FIG. 1 in that ceramic substrate 4 is not formed with arecess for use in receiving IC chip 2, but has a flat shape.Particularly, one of main surfaces of ceramic substrate 4, on which ICchip 2 is mounted, is flat. Here, flat ceramic substrate 4 is made of asingle ceramic layer. Mounting electrodes 7 are formed at four cornerson the back surface of ceramic substrate 4 for use in mounting thecrystal oscillator on a wiring board. Four mounting electrodes 7correspond to the power supply, ground, oscillation output, and AFCterminals among the IC terminals, respectively.

This crystal oscillator uses a pair of metal supporters 20 for disposingcrystal blank 3 above IC chip 2. Supporters 20 each have a height largerthan the thickness of IC chip 2, and is made up of a body extendingvertically, and L-shaped members disposed at both ends of the body,respectively. The L-shaped members at one end (i.e., proximal end) andthe other end (i.e., distal end) of supporter 20 have their leading endsextending in the same direction. The L-shaped member at the proximal endhas a side surface bonded to relay terminal 17 formed on the top surfaceof ceramic substrate 4. The upper end side of supporter 20 protrudesupward from the top surface of IC chip 2. Substantially rectangularcrystal blank 3 is horizontally held above IC chip 2 by securing a pairof lead-out electrodes extending from both ends on one side of crystalblank 3 to side surfaces of the L-shaped members at the distal ends ofthe pair of supporters 20 with a conductive adhesive (not shown) in sucha manner that the longitudinal direction of crystal blank 3 matches thelongitudinal direction of ceramic substrate 4. Here, crystal blank 3 hasa length larger than the length of IC chip 2 in the same direction, sothat IC chip 2 is completely covered with crystal blank 3. Of course,crystal blank 3 can be formed smaller than IC chip 2, but larger crystalblank 3 results in a larger plate area which facilitates the designingfor accomplishing desired oscillation characteristics.

FIG. 9 illustrates the top surface of ceramic substrate 4. Metal film 9,which is formed along each side of substantially rectangular ceramicsubstrate 4, is formed closer to the center of ceramic substrate 4 inthe four corner portions of ceramic substrate 4 such that metal film 9traces circular arcs at the corner portions. Then, at the four corners,end face electrodes 7 b are formed on the side surfaces of ceramicsubstrate 4 by through-hole processing. End face electrodes 7 b aredisposed integrally with associated mounting electrodes 7.

As described above, circuit terminals 6 and a pair of relay terminals 17are formed on the top surface of ceramic substrate 4. On illustratedceramic substrate 4, a total of eight circuit terminals 6 are disposed,four along each of the long sides of ceramic substrate 4. These circuitterminals, which correspond to the IC terminals of IC chip 2, include apair of connection terminals 6(X1), 6(X2) for connection to crystalblank 3, power supply terminal 6(Vcc), oscillation output terminal6(Vout), ground terminal 6(E), AFC terminal 6(AFC), and a pair of writeterminals 6(W1), 6(W2) for writing temperature compensation data. Amongthese circuit terminals, connection terminals 6(X1), 6(X2) areelectrically connected to relay terminals 17 through conductive pathsformed on the surface of ceramic substrate 4. Power supply terminal6(Vcc), oscillation output terminal 6(Vout), ground terminal 6(E), andAFC terminal 6(AFC) are connected to corresponding mounting electrodes 7through conductive paths formed on the surface of ceramic substrate 4and via-holes 19 extending through ceramic substrate 4, respectively.

Write surface terminals 7(W1), 7(W2) are disposed on the back surface ofceramic substrate 4, other than the aforementioned mounting electrodes7, for electric connection to write terminals 6(W1), 6(W2),respectively. In FIG. 9, the mounting electrodes and write surfaceelectrodes are formed at positions indicated by broken lines. Each ofwrite surface terminals 7(W1), 7(W2) is placed at the center betweenmounting electrodes 7, which are disposed at both ends on a long side ofceramic substrate 4, along the long side. A probe for writing data isaccessed to write surface terminals 7(W1), 7(W2) for writing temperaturecompensation data into the temperature compensation mechanism within ICchip 2. Write surface terminals 7(W1), 7(W2) are electrically connectedto corresponding write terminals 6(W1), 6(W2), respectively, throughconductive paths routed on the surface of ceramic substrate 4 andvia-holes 19 extending through ceramic substrate 4. Further, for settingmetal cover 5 at a ground potential, metal film 9 on ceramic substrate 4is also electrically connected to mounting electrode 7 corresponding tothe ground terminal through via-hole 19. In this way, so-called casegrounding is performed.

Ceramic substrate 4 is formed by burning a green ceramic sheet (i.e.,unburned ceramic sheet). Via-holes 19 are formed by providingthrough-holes through the green ceramic sheet, filling the through-holeswith a printing material, and disposing a plating layer on a circuitpattern after burning when a circuit underlying pattern (i.e., circuitterminals 6, relay terminals 17, conductive paths 18, and mountingelectrodes 7) is formed on the ceramic sheet by printing. The printingmaterial used herein may be, for example, molybdenum (Mo) or tungsten(W). The plating layer is formed by laminating a nickel (Ni) layer and agold (Au) layer. By forming the plating layer after the burning,via-holes 19 are completed, and in this event, the through-holes areclosed to maintain hermetic package 1 in an air-tight state.

In this crystal oscillator, ceramic substrate 4 has both main surfacesformed flat, and is therefore free from a problem of non-uniformcontraction which would be otherwise experienced when a second ceramiclayer having an opening is laminated on a flat first ceramic layer, andthe resulting laminate is burned. Consequently, in this embodiment,ceramic substrate 4 remains flat even after burning, without beingcurved. Thus, when IC chip 2 is secured to ceramic substrate 4 throughultrasonic thermo-compression bonding using bumps 12, the IC terminalsand circuit terminals 6 establish secure electric connectionstherebetween. The use of concave metal cover 5 facilitates a reductionin size of the surface mount type crystal oscillator, as is the casewith the conventional one illustrated in FIG. 1. Also, since ceramicsubstrates can be formed of a single ceramic layer, the manufacturingcost can be reduced.

It should be noted that write surface terminals 7(W1), 7(W2) disposed onthe bottom surface of ceramic substrate 4 can be used as mountingelectrodes as required in order to enhance a connection strength to awiring board.

Next, a surface mount type crystal oscillator according to a secondembodiment of the present invention will be described with reference toFIGS. 10 to 12.

In the crystal oscillator illustrated in FIG. 8, the mounting electrodesand surface write terminals are disposed at the same level on the backsurface of ceramic substrate 4. In such a configuration, the surfacewrite terminals can be used for mounting like the mounting electrodes,whereas a circuit pattern on a wiring board can unintentionally comeinto electric contact with the surface write terminals. To avoid suchunintentional electric contacts, the crystal oscillator of the secondembodiment has surface write terminals 7(W1), 7(W2) formed in associateddimples, as illustrated in FIG. 10, such that mounting electrodes 7 aredifferent in level from surface write terminals 7(W1), 7(W2), morespecifically, such that mounting electrodes 7 come into contact with awiring board, but surface write terminals 7(W1), 7(W2) do not come intocontact with the wiring board when the crystal oscillator is carried onthe wiring board.

As illustrated in FIG. 10, the crystal oscillator of the secondembodiment is similar to the crystal oscillator of the first embodimentillustrated in FIG. 8, but differs from the first embodiment in thatsubstantially rectangular ceramic substrate 4 is made of a laminatedceramic substrate composed of first ceramic layer 4 a and second ceramiclayer 4 b, and cutout portions 21 are defined in first ceramic layer 4 awhich is placed on the bottom side.

Specifically, second ceramic layer 4 b has a flat top surface on whichcircuit terminals 6, metal film 9, and relay terminals 17 are formed.Mounting electrodes 7 are formed at four corners on the back surface offirst ceramic layer 4 a. Then, as illustrated in FIG. 11, cutoutportions 21 are defined in substantially rectangular areas along therespective long sides of first ceramic layer 4 a. Surface writeterminals 7(W1), 7(W2) are formed on the back surface of second ceramiclayer 4 b, i.e., on the surface of second ceramic layer 4 b which islaminated on first ceramic layer 4 a, at the positions of cutoutpositions 21 in first ceramic layer 4 a. Since there is not firstceramic layer 4 a in cutout portions 21, ceramic substrate 4 composed offirst ceramic layer 4 a and second ceramic layer 4 b laminated thereoncauses surface write terminals 7(W1), 7(W2) on second ceramic layer 4 bto expose at the positions of cutout portions 21, as illustrated in FIG.12, when viewed from the back side.

It should be noted that end face electrodes 7 b are disposed at fourcorners of first ceramic layer 4 a on end faces of first ceramic layer 4a in a manner similar to the foregoing, but illustration of end faceelectrodes 7 b is omitted in FIG. 12 for convenience.

Mounting electrodes 7 at the four corners are electrically connected topower supply terminal 6(Vcc), output terminal 6(Vout), ground terminal6(E), and AFC terminal 6(AFC) disposed on the top surface of secondceramic layer 4 b, respectively, through via-holes and conductive pathsrouted on the laminated surface. Likewise, write surface terminals7(W1), 7(W2) are electrically connected to write terminals 6(W1), 6(W2),respectively, through via-holes and conductive paths.

In the configuration as described above, even though cutout portions 21are formed in central areas of both sides of first ceramic layer 4 a,cutout portions 21 have a smaller area than the opening formed throughthe second ceramic layer of the conventional crystal oscillatorillustrated in FIG. 1, and are positioned along the outer periphery offirst ceramic layer 4 a, so that even if an external force acts due to acontractive force during burning, the contractive force merely exerts asmall influence on general curvature of ceramic substrate 4. In otherwords; even such cutout portions would not lead to curvature of theceramic substrate. Accordingly, electric connections are establishedbetween the IC terminals and circuit terminals 6 without fail when ICchip 2 is secured to ceramic substrate 4 through ultrasonicthermo-compression bonding using bumps 12, in a manner similar to thefirst embodiment. In this embodiment, first ceramic layer 4 a has athickness of 50 to 200 mm, for example, and preferably 80 to 100 μm.

In the second embodiment, since write surface terminals 7(W1), 7(W2) areformed in dimples defined by cutout portions 21, write surface terminals7(W1), 7(W2) are prevented from coming into contact with a wiringpattern on a wiring board when this crystal oscillator is mounted on thewiring board. Also, sine write surface terminals 7(W1), 7(W2) aredisposed in cutout portions 21 formed along the outer periphery ofceramic substrate 4, a probe for writing temperature compensation datacan be more readily accessed to write surface terminals 7(W1), 7(W2)than when write-surface terminals 7(W1), 7(W2) are disposed in a recessformed in a central area of a substrate, thus leading to improvedoperability.

In this embodiment, write surface terminals 7(W1), 7(W2) for writingtemperature compensation data are placed in correspondence to cutoutportions 21 in first ceramic layer 4 a, but write surface terminals7(W1), 7(W2) may be replaced with a pair of crystal test terminals 7Q1,7Q2 which are electrically connected to a pair of relay terminals 17through conductive paths, not shown. The use of crystal test terminals7Q1, 7Q2 permits the electric characteristics and oscillationcharacteristics of crystal blank 3, as a crystal element, to beindependently measured from another circuit even after crystal blank 3is encapsulated in hermetic package 1. When a single write terminal isprovided in IC chip 2, cutout portion 21 is only required along one longside of first ceramic layer 4 a, as a matter of course.

Next, a description will be given of a surface mount type crystaloscillator according to a third embodiment of the present invention.While the crystal oscillator of the second embodiment comprises twowrite surface terminals, three or more write surface terminals may berequired depending on the configuration of IC chip 2, or some IC chipsare preferably provided with both write surface terminals and crystaltest terminals. The crystal oscillator of the third embodiment can beprovided with up to four write surface terminals and/or crystal testterminals in combination.

The crystal oscillator of the third embodiment is similar to the crystaloscillator of the second embodiment but differs in the position ofcutout portions in first ceramic layer 4 a. As illustrated in FIG. 13,first ceramic layer 4 a used in the crystal oscillator of the thirdembodiment is formed with substantially rectangular cutout portions 21in central areas along respective sides of first ceramic layer 4 a.These cutout portions 21 are all positioned between mounting electrodes7.

By thus forming four cutout portions 21, it is possible to provide atotal of four write surface terminals and/or crystal test terminals incombination. Then, as illustrated in FIG. 14A, four write surfaceterminals 7(W1) to 7(W4) can be formed on the surface of second ceramiclayer 4 b exposed through cutout portions 21. According to thisconfiguration, it is possible to use IC chip 2 which requires four writeterminals. Alternatively, as illustrated in FIG. 14B, write surfaceterminals 7(W1), 7(W2) can be disposed in cutout portions 21 associatedwith one set of two opposing sides, while crystal test terminals 7Q1,7Q2 can be disposed in cutout portions 21 associated with the other setof two opposing sides. In this way, since both the write surfaceterminals and crystal test terminals can be formed on the bottom surfaceof ceramic substrate 4, the same measuring tools can be convenientlyused by way of example.

The third embodiment is similar to the first and second embodiments inthat cutout portions 21 are formed in an outer peripheral area of firstceramic layer 4 a, and IC chip 2 is mounted on a flat central area, sothat ceramic substrate 4 is prevented from being curved even if acontractive force acts thereon during burning. Also, since the cutoutportions are located symmetrically about the center line of ceramicsubstrate 4, no excessive stress will act on ceramic substrate 4.

Next, a description will be given of a surface mount type crystaloscillator according to a fourth embodiment of the present invention. Inthe surface mount type crystal oscillators of the second and thirdembodiments, a step is formed on the back surface of ceramic substrate 4and the mounting electrodes and write surface terminals are disposedwith the step intervening therebetween. Such a configuration can be alsoapplied to the conventional crystal oscillator illustrated in FIG. 4.FIG. 15 illustrates the crystal oscillator illustrated in FIG. 4 whichis additionally provided with a step formed on the bottom surface ofpackage-body 1 a made of laminated ceramic layers and comprises mountingelectrodes 7 and write surface terminals 7(W1), 7(W2) disposed with thestep intervening therebetween. In the illustrated crystal oscillator,among ceramic layers which make up package body 1 a, lowermost ceramiclayer 1 c, as viewed in the figure, is formed with cutout portions 21,such that write surface terminals 7(W1), 7(W2) are disposed on a surfacewhich is exposed through cutout portions 21.

Next, a description will be given of a surface mount type crystaloscillator according to a fifth embodiment of the present invention. Thecrystal oscillator of the fifth embodiment illustrated in FIG. 16 issimilar to the first embodiment illustrated in FIG. 8, but differs in amethod of holding crystal blank 3 in hermetic package 1.

In the crystal oscillator illustrated in FIG. 16, IC chip 2, which isthe same as that in the first embodiment, is secured to the top surfaceof ceramic substrate 4 through ultrasonic thermo-compression bondingusing bumps 12 by bonding IC terminals of IC chip 2 to circuit terminals6 formed on the top surface of ceramic substrate 4. Circuit terminals 6,metal film 9, and relay terminals 17 on ceramic substrate 4 areidentical in position to those illustrated in FIG. 9. Crystal blank 3 isan AT-cut quartz crystal blank as shown in FIG. 3. The crystal blank isheld by the supporters in the first embodiment, whereas in the fifthembodiment, crystal blank 3 is held in hermetic package 1 by securingone side of crystal blank 3, from which lead-out electrodes 14 areextended, to the back surface (i.e., a main surface opposite to thecircuit forming surface) of IC chip 2 through insulating adhesive 29, Inother words, insulating adhesive 29 intervenes between an end region ofthe main surface of crystal blank 3 opposing IC chip 2 and IC chip 2.Then, relay terminals 17 disposed on the top surface of ceramicsubstrate 4 are electrically connected to lead-out electrodes 14 on thetop surface of crystal blank 3, as viewed in FIG. 16, by wire bondingusing gold wires 30. In this crystal oscillator, crystal blank 3 has alongitudinal length shorter than the length of IC chip 3 in the samedirection, such that a leading portion of crystal blank 3 does notprotrude outward from the outer peripheral contour of IC chip 2. Theremaining aspects of the crystal oscillator of the fifth embodiment arethe same as the crystal oscillator of the first embodiment.

Like the first embodiment, the crystal oscillator of the fifthembodiment can be manufactured at a reduced cost by virtue of the use ofa ceramic substrate made of a single layer. Then, since crystal blank 3is directly secured on IC chip 2, no step is needed to be formed in theceramic substrate for holding crystal blank 3, so that ceramic substrate4 can be reduced in size, as compared with the conventional oneillustrated in FIG. 1. Stated another way, a ceramic substrate of thesame size can provide a larger internal volume which can be effectivelyutilized within hermetic package 1. For example, the distance from oneend of IC chip 2 to the inner peripheral surface of metal cover 5 can bereduced to approximately 0.3 mm. On the other hand, in the conventionalcrystal oscillator illustrated in FIG. 1, a length of approximately 0.4mm is required for the step because an area is required for applying anadhesive, and a gap of approximately 15 mm is additionally required formounting IC chip 2 in the recess. Thus, from the fact that theconventional crystal oscillator requires approximately 0.55 mm extra,the length of ceramic substrate 4 can be reduced by 0.25 mm in thisembodiment, as compared with the one illustrated in FIG. 1.

Also, since crystal blank 3 has its one side directly secured on IC chip2 with insulating adhesive 29 so that IC chip 2 additionally serves as aholder for crystal blank 3, the number of parts can be reduced.

While crystal blank 3 is horizontally placed in the crystal oscillatorillustrated in FIG. 16, crystal blank 3 may be inclined upward towardthe opposite side thereof to prevent crystal blank 3 from coming intocontact with IC chip 2. In this event, as long as the opposite side ofcrystal blank 3 rises up by a height approximately equal to a loop ofgold wire 30 used for wire bonding, the crystal oscillator can maintaina low profile.

Next, a description will be given of a surface mount type crystaloscillator according to a sixth embodiment of the present invention. Inthe fifth embodiment described above, crystal blank 3 is directlysecured to IC chip 2 with an insulating adhesive, whereas in the crystaloscillator of the sixth embodiment illustrated in FIG. 17A, crystalblank 3 is mounted on IC chip 2 through the intervention of auxiliarysubstrate 22. The rest of the configuration in the sixth embodiment isthe same as the fifth embodiment. Auxiliary substrate 22 is made, forexample, of a crystal plate having the same cutting angle (i.e., AT-cut)as crystal blank 3, and is provided with a pair of crystal holdingterminals 10 on one main surface thereof. Auxiliary substrate 22 issecured to the back surface of IC chip 2 with insulating adhesive 29,while crystal blank 3 is secured to auxiliary substrate 22 withconductive adhesive 15. FIG. 17B illustrates a positional relationshipbetween auxiliary substrate 22 and crystal blank 3. A pair of crystalholding terminals 10 are electrically connected to a pair of relayterminals 17 on ceramic substrate 4 by wire bonding using gold wires 30.

A description will now be given of processes of manufacturing thecrystal oscillator as described above. First, IC chip 2 is secured onceramic substrate 4 through ultrasonic thermo-compression bondingsimilar to the foregoing, and auxiliary substrate 21 is secured on ICchip 2 with insulating adhesive 19. Next, both ends on one side ofcrystal blank 3, from which lead-out electrodes 14 extend, are securedto given end areas of crystal holding terminals 10 on auxiliarysubstrate 22 with conductive adhesive 15. Finally, opposite end areas ofcrystal holding terminals 10 are connected to relay terminals 17 onceramic substrate 4 with gold wires 30 for wire bonding, and the openingend face of metal cover 5 is bonded to metal film 9 disposed around theouter periphery of ceramic substrate 4 through thermo-compressionbonding using brazing metal 11 for sealing.

In the foregoing configuration, auxiliary substrate 22 is entirelysecured on IC chip 2 and is therefore mechanically stable, thusfacilitating wire bonding operations for connecting crystal holdingterminals 10 to relay terminals 17. The intervening auxiliary substrate21 causes a relatively large gap between crystal blank 3 and IC chip 2to facilitate securing operations and the like and to prevent crystalblank 3 from coming into contact with IC chip 2 to maintain goodoscillation characteristics. Since auxiliary substrate 22 is made of acrystal plate having the same cutting angle (i.e., AT-cut) as crystalblank 3, auxiliary substrate 22 and crystal blank 3 have the samecoefficient of thermal expansion, thus eliminating distortions due to adifference in the coefficient of thermal expansion. Consequently, thecrystal element (crystal blank 3) is improved in thermal shockresistance characteristics.

Since the sixth embodiment also employs ceramic substrate 4 made of asingle layer, as is the case with the fifth embodiment, the crystaloscillator can be manufactured at a reduced cost. Also, since no step isneeded to be formed in the ceramic substrate for holding the crystalblank, ceramic substrate 4 can be reduced in length to increase theeffective internal volume of sealed package 1.

In the fifth and sixth embodiments described above, ceramic substrate 4is made of a single layer, but alternatively may be made of laminatedceramics. In this event, since green ceramic sheets need not be stamped,the manufacturing cost can be restrained from increasing. When laminatedceramics are used, conductive paths can be formed on laminated surfacefor electrically connecting IC chip 2 to mounting electrodes 7 to ensurethe air-tight sealing of the hermetic package. When a temperaturecompensated crystal oscillator is implemented, ceramic substrate 4 maybe formed with terminals for writing temperature compensation data,terminals for testing characteristics of a crystal element, and the likeon side surfaces thereof. Electrode patterns for write terminals andcharacteristic test terminals may not be formed on a lowermost ceramiclayer and an uppermost ceramic layer to prevent these terminals fromelectrically shorting, for example, with a conductor pattern on a wiringboard.

In the first to sixth embodiments described above, end face electrodes 7b may not be disposed at four corners of ceramic substrate 4 bythrough-hole processing, such that metal film 9 is formedcorrespondingly closer to the edges of ceramic substrate 4 to increasethe substantial internal volume of hermetic package 1. Metal film 9 maybe slightly spaced apart from the side edges of ceramic substrate 4 inorder to facilitate a process of dividing burned laminate of ceramicsheets into individual ceramic substrates 4. Also, in each of theembodiments described above, the crystal oscillator has been describedto be a temperature compensated crystal oscillator into whichtemperature compensation data is written from the outside, the crystaloscillator may be an SPXO (simple packaged crystal oscillator) which isnot equipped with the temperature compensation mechanism. Further,crystal blank 3 disposed above IC chip 2 has a free end from whichlead-out electrodes 14 do not extend, so that an impact, if applied,would cause the free end to collide with the back surface of IC chip 2,possibly resulting in deteriorated oscillation characteristics.Accordingly, a flexible adhesive may be applied to the back surface ofIC chip 2 or to the inner surface of metal cover 5, corresponding to thefree end of crystal blank 3, so as to reduce a width over which crystalblank 3 swings when an impact is applied thereto.

Next, a description will be given of crystal devices according to thepresent invention. FIG. 18A is a cross-sectional view illustrating theconfiguration of a surface mount type crystal unit which is an exampleof crystal devises according to the present invention.

The crystal unit illustrated in FIG. 18A is similar to the conventionalcrystal unit illustrated in FIG. 6, but differs from that illustrated inFIG. 6 in that pier 23 is disposed along the inner periphery of metalfilm 9 on the top surface of ceramic substrate 4 to form a step againstthe outer periphery portion. Step width W1 of the step is larger thanframe width W2 of metal cover 5 including a flange on an open end faceof metal cover 5. Then, pier 23 has a height (thickness) larger than atotal thickness of metal film 9 and eutectic alloy 11.

In this example, metal film 9 is also produced by forming underlyingelectrodes of tungsten (W) or molybdenum (No) on a green ceramic sheetby printing, burning the ceramic sheet, and subsequently laminating anNi layer and an Au layer on the underlying electrodes by plating. Then,pier 23 is formed when the underlying electrodes are formed, by printingmultiple layers of W or Mo, which is the same material as the underlyingelectrodes, only on the inner peripheral side of the underlyingelectrodes. After burning, an Ni and an Au layer are laminated on thesurface including the side surface of pier 23 by plating in a mannersimilar to metal film 9.

In this embodiment, after crystal blank 3 has been secured to crystalholding terminals 10, the open end face of metal cover 5 including theflange is brought into contact with eutectic alloy 11 disposed on metalfilm 9, and metal cover 5 is bonded to ceramic substrate 4 by meltingeutectic alloy 11 by heating, thereby hermetically sealing crystal blank3 and completing the crystal unit.

In the configuration as described above, the step formed by pier 23 onthe inner peripheral side of metal film 9 can prevent metal cover 5 fromshifting in position when it is bonded by eutectic alloy 11. It istherefore possible to ensure a sufficient seal-path between metal cover5 and ceramic substrate 4 to increase the bonding strength and air-tightsealing. Particularly, in this example, since the Ni layer and Au layerare laminated on the top and side surfaces of pier 23 in a mannersimilar to metal film 9, eutectic alloy 11 introduces into a gap betweenmetal cover 5 and pier 23 when it is melted by heating for bonding. Inthis way, a bonding area with eutectic alloy 11 extends to increase theseal-path, resulting in further increased bonding strength and air-tightsealing. Conversely, since the bonding strength and air-tight sealingcan be kept high, the flange may be protruded by a shorter length onmetal cover 5 to substantially increase the internal volume of thehermetic package.

Also, since frame width W2 of the opening end face of metal cover 5 ismade smaller than W2 in consideration of a clearance for step width W1of the step, the outer periphery of metal cover 5 will not protrudeoutward from ceramic substrate 4. Consequently, the surface mount typecrystal unit satisfies criteria for outer dimensions without fail.

In the foregoing example, pier 23 for forming the step is created byprinting multiple layers of the same materials as the underlyingelectrodes, but pier 23 may be made of an insulating material such asalumina instead of the materials of the underlying electrodes. Whilepier 23 is formed to make a round on ceramic substrate 4 along metalfilm 9, the pier 23 is not necessarily so formed. Pier 23 may be locallyformed on each of four sides of ceramic substrate 4. Alternatively,L-shaped piers may be formed at a pair of corners on both sides of onediagonal of ceramic substrate 4. Pier 23 may be disposed in whichevermanner as long as it has a height larger than the total thickness ofmetal film 9 and eutectic alloy 11 to have the ability to position metalcover 5.

From a viewpoint of the positioning of metal cover 5, any step may beformed, and the step need not be created by a pier. FIG. 18B illustratesceramic substrate 4 composed of laminated ceramic layers, the secondlayer of which is formed smaller than the first layer to create a step,instead of forming a pier. In this event, metal film 9 may be formed onan exposed surface of the first ceramic layer. Such ceramic substrate 4can be formed by laminating a second green ceramic sheet divided intoindividual crystal oscillators on a first green ceramic sheet having anarea corresponding to a plurality of crystal oscillators, burning theresulting laminate, and dividing the first ceramic sheet.

Alternatively, a single green ceramic sheet may be formed with mark-offlines, which run vertically and horizontally, by embossing, burned, andthen cut vertically and horizontally along the mark-off lines forformation into ceramic substrates 4, each of which has a step on theouter periphery, as illustrated in FIG. 18C. In this event, crystalholding terminals 10 for holding crystal blank 3 are connected tomounting electrodes 7 through linear via-holes which extend throughceramic substrate 4, thereby maintaining the air-tight sealing.

In the configurations illustrated in FIGS. 18B and 18C, the step ispreferably made to have larger width W1 to prevent mounting electrodes 7from short-circuiting with metal film 9. However, for facilitating areduction in size, the crystal unit may not include end face electrodes,each of which extends from associated mounting electrode 7.

The bonding of metal cover 5 to ceramic substrate 4 is not limited to amethod which is based on a eutectic alloy. For example, as illustratedin FIG. 19A, the bonding may be carried out by seam welding whichinvolves bringing electrode rollers 24 a, 24 b into contact with theflange of metal cover 5, and bonding metal cover 5 to metal film 9.Alternatively, as illustrated in FIG. 19B, the bonding may be carriedout by electron beam welding which involves irradiating the flange withelectron beam P to bond metal cover 5 to metal film 9. Since neither theseam welding nor electron beam welding uses a eutectic alloy, themanufacturing cost can be more reduced for mass-produced items.

While the crystal devices based on the present invention have beendescribed in connection of exemplary crystal units, each of which has acrystal blank hermetically sealed in a package, the present inventioncan also be applied to surface mount type crystal oscillators. FIG. 20illustrates that metal cover 5 can be positioned by piers 23 provided inthe aforementioned crystal oscillator illustrated in FIG. 8.

1. A surface mount type crystal oscillator comprising a crystal blank,an IC chip having at least an oscillation circuit using said crystalblank, integrated thereon, and a hermetic package for accommodating saidcrystal blank and said IC chip therein, wherein: said hermetic packagecomprises a ceramic substrate having a substantially rectangular shapeand formed with a metal film which makes a round on one main surfacethereof, and a concave metal cover having an open end face bonded tosaid metal film, said ceramic substrate comprises mounting electrodes atcorners on the other main surface of said ceramic substrate for use inmounting said crystal oscillator on a wiring board, said IC chip issecured to the one main surface of said ceramic substrate throughultrasonic thermo-compression bonding using bumps, said crystal blank isdisposed above said IC chip, and said one main surface of said ceramicsubstrate is formed as a flat surface.
 2. The crystal oscillatoraccording to claim 1, wherein said other main surface of said ceramicsubstrate is formed as a flat surface.
 3. The crystal oscillatoraccording to claim 1, further comprising a supporter having one endbonded to the one main surface of said ceramic substrate, and the otherend adapted to support said crystal blank thereon.
 4. The crystaloscillator according to claim 1, wherein: said ceramic substratecomprises a first ceramic layer formed with said mounting electrodes,and a second ceramic layer laminated on said first ceramic layer forsecuring said IC chip thereto, said first ceramic layer comprises acutout portion in a central area on at least one side thereof, saidcutout portion having an outer periphery open to said side, and anadjustment terminal is formed on a laminated surface of said secondceramic layer exposed by said cutout portion.
 5. The crystal oscillatoraccording to claim 4, wherein: said adjustment terminal includes a writesurface terminal used to write temperature compensation data into atemperature compensation mechanism within said IC chip, and/or a crystaltest terminal used to measure oscillation characteristics of saidcrystal blank.
 6. The crystal oscillator according to claim 4, whereinsaid first ceramic layer comprises said cutout portion in each ofcentral areas of at least one pair of opposing sides thereof.
 7. Asurface mount type crystal oscillator comprising a crystal blank, an ICchip having at least an oscillation circuit using said crystal blank,integrated thereon, and a package having at least a substantiallyrectangular ceramic substrate and adapted to accommodate said crystalblank and said IC chip therein, wherein: said ceramic substratecomprises mounting electrodes at corners on a surface thereof which isan outer bottom surface of said package for use in mounting said crystaloscillator on a wiring board, said ceramic substrate comprises a firstceramic layer formed with said mounting electrodes, and a second ceramiclayer laminated on said first ceramic layer for securing said IC chipthereto, said first ceramic layer comprises a cutout portion in acentral area on at least one side thereof, said cutout portion having anouter periphery open to said side, and an adjustment terminal is formedon a laminated surface of said second ceramic layer exposed by saidcutout portion.
 8. The crystal oscillator according to claim 7, wherein:said adjustment terminal includes a write surface terminal used to writetemperature compensation data into a temperature compensation mechanismwithin said IC chip, and/or a crystal test terminal used to measureoscillation characteristics of said crystal blank.
 9. The crystaloscillator according to claim 7, wherein said first ceramic layercomprises said cutout portion in each of central areas of at least onepair of opposing sides thereof.
 10. The crystal oscillator according toclaim 7, wherein said first ceramic layer comprises said cutout portionin a central area of each side thereof, and said adjustment terminalsinclude a pair of write surface terminals used to write temperaturecompensation data into a temperature compensation mechanism within saidIC chip, and a pair of crystal test terminals used to measureoscillation characteristics of said crystal blank. 11-20. (canceled)